Showing papers on "Pulse-frequency modulation published in 1994"

Chirp signal matching and signal power optimization in pulse-echo mode ultrasonic nondestructive testing

Abstract: Chirp pulse compression is a signal correlation technique that uses frequency modulated pulses as transmitted signals. Usually, signals with linear frequency modulation are applied. They can be generated rather easily, but their spectra are not totally matched to the transfer function of ultrasonic systems. In pulse-echo mode operation, with signal duration and consequently the time-bandwidth product being critical parameters, waveforms should be applied which make full use of the available power and bandwidth resources. We report here two methods to improve the overall efficiency of an ultrasonic pulse-echo system. Transmitter signals with constant amplitude level and nonlinear frequency modulation can be generated in such a way that they are spectrally matched to the system. A formula for the calculation of such a matched nonlinear chirp signal is presented. This modulation scheme also leads to a side-lobe level reduction of the compressed pulses. The application of square wave chirps derived from sine type chirps yields an additional gain of echo signal amplitude. Moreover, the complexity of the signal generation hardware is reduced. The methods are illustrated by an example. >

Symmetry control circuit and method

Abstract: The present invention provides a method and circuit for controlling the flow of current through a load. In a preferred embodiment, an oscillator generates a pulse signal of constant frequency. A pulse width modulator adjusts the duty cycle of the pulse signal in response to a dimming level signal input indicative of the desired level of current flow through the load. A converter receives the pulse signal as an input and converts it into an AC signal, the frequency of which follows the frequency of the pulse signal and the symmetry of which varies with the duty cycle of the pulse signal. The load is connected into a resonant circuit tuned such that a change in the symmetry of the AC signal changes the level of current flowing through the load.

Generation of optical signals with rf components

Abstract: An optical beam having a high radio-frequency modulation is generated at output (10) by generating in source (4a) a lower frequency modulation, using it to control the optical output (3) of a laser (1) and further modulating the optical output (3) in an optical modulator (6) by a control signal having another lower frequency modulation generated by source (4b). Either or both of the lower frequency modulations also carries an information containing modulation. The effect of the optical modulator (6) is to up-convert the modulation carried by the optical beam by the modulation frequency of the control signal. The optical modulator (6) may be a Mach-Zehnder interferometer. The non-linearity of such a modulator with respect to its control input may be exploited by selecting the amplitude of the control signal such that the optical output (3) is upconverted by an integer multiple of the modulation frequency generated by source (4b). These methods avoid the need to apply the high frequency modulation to either the laser input (2) or the control input directly (7).

Effects of carrier frequency and background noise on the detection of mixed modulation

Abstract: This article is concerned with the mechanisms underlying the detection of amplitude modulation (AM), frequency modulation (FM), and mixed modulation (MM), i.e., simultaneously occurring AM and FM. In a previous study [B. C. J. Moore and A. Sek, J. Acoust. Soc. Am. 92, 3119–3131 (1992)], psychometric functions were measured for the detection of AM alone and FM alone, using a 10‐Hz modulation rate and a 1‐kHz carrier frequency. Detectability was then measured for combined AM and FM, with modulation depths selected so that each type of modulation would be equally detectable if presented alone. The detectability of the combined AM and FM was better than would be predicted if the two types of modulation were coded completely independently. Significant effects of relative modulator phase were found when detectability was relatively high, but these effects were not correctly predicted by either of two excitation‐pattern models considered. The first experiment reported here was similar to the earlier experiment, ...

System and method for modulating a carrier frequency

Abstract: A carrier frequency modulating system and method for producing a modulated carrier frequency is disclosed. The system comprises a carrier frequency signal generator for producing a carrier frequency signal at a carrier frequency at an output thereof. The system further comprises a sequencer for producing a modulation signal at an output thereof. This modulation signal comprises a plurality of modulation signal portions separated by at least one null modulation signal portion having a corresponding time length. The system is provided with a modulator for producing a modulated carrier frequency signal at an output thereof. The modulator has a first input connected to the output of the carrier frequency signal generator for receiving the carrier frequency signal, and has a second input for receiving the modulation signal. The system further comprises a power supply unit for supplying electrical power to the carrier frequency signal generator. The sequencer is connected to the carrier frequency signal generator for controlling the activation thereof and to interrupt the carrier frequency signal generator instead of modulating the null modulation signal portion during the corresponding time length, whereby power consumption of the system is minimized.

Short range inductively coupled communication system employing time variant modulation

Abstract: A short range magnetically coupled wireless communication system (fig. 1) employs time variant modulation of a high repetition rate pulse stream and magnetic coupling between a transmission magnetic element and a receiving magnetic element (16). The pulse stream is modulated by an input audio frequency signal in the time domain, for example through pulse position modulation (2A), pulse width modulation or pulse symmetry modulation (2B). The receiving magnetic element is coupled to a demodulator circuit which detects the transmitted pulses induced in the receiving magnetic element and reproduces the audio frequency signal (fig. 6). Transmission over short range is thus efficiently accomplished through magnetic coupling in a simple low cost low power consumption communication system.

Electro-optic modulator having gated-dither bias control

Abstract: An electro-optic modulator includes a closed loop bias control system for maintaining the modulator at its half-power point by modulating a time varying signal on and off at a modulation frequency fm. The modulation frequency fm is much lower than the frequency components within the spectrum of the time varying signal, such that, energy within the time varying signal spectrum is recovered at the modulation frequency when the modulator is not operating at the half-power point. A phase sensitive demodulator operating at the modulation frequency fm demodulates a signal indicative of the modulator optical output signal to provide a DC voltage signal value indicative of the half-power point bias error value. The time varying voltage signal may be a dither noise signal or any other time varying signal such as a signal with a single frequency component (e.g. a sine or cosine) or a signal with a plurality of frequency components (e.g., a square wave).

Amplitude modulation rate discrimination with sinusoidal carriers

Abstract: Discrimination of the change in rate of sinusoidal amplitude modulation was investigated for sinusoidal carriers. The just-noticeable change in the modulation rate was measured as a function of (1) carrier frequency (fc = 500, 1000, 2000, or 4000 Hz), (2) initial modulation rate (fm = 10, 20, 40, 80, 160, or 320 Hz), and (3) stimulus duration (T = 62, 125, 250, 500, 1000, or 2000 ms). For modulation rates less than 320 Hz, the thresholds for a change in modulation rate were not affected by changing the carrier frequency. When fm = 320 Hz, the threshold for changes in the modulation rate ordered in the same way as the threshold for changes in the frequency of a pure tone, suggesting that the primary cue for discriminating a change in modulation rate was the frequency of either sideband. As stimulus duration increased to a certain T (critical duration), the thresholds decreased by more than a factor of 2. Above that duration, the thresholds for changes in modulation rate decreased only slightly or remained constant. The critical duration corresponded to about five cycles of modulation, at least for the slower modulation rates.

Spectral analysis of the sinusoidal PWM with variable switching frequency for noise reduction in inverter-fed induction motors

Abstract: The first part of the paper describes the relationship between inverter output voltage and radial magnetic force on the inner surface of the stator, which generates vibrations of the stator and causes the magnetic noise. The theoretical results are verified experimentally. In the second part the authors derive the frequency components of the pulse width modulated (PWM) and pulse frequency modulated (PFM) voltage and of the magnetic noise. This is the basis for a calculable spreading of the spectrum by means of PFM, shown in the third part. The purpose is noise reduction and elimination of selected harmonics to avoid mechanical resonances. >

Pulse width modulation speaker amplifier

Abstract: An open-loop speaker amplifier and a dynamic range reduction circuit are disclosed. The speaker amplifier includes a bridge driver and a bridge circuit. The bridge circuit includes a first set of switches and a second set of switches. When the first set of switches are activated, a positive current flows through a speaker load. When the second set of switches are activated, a negative current flows through the speaker load. The bridge driver generates a pulse width modulated signal and a delayed pulse width modulated signal to drive the first and second set of switches of the bridge circuit responsive to a modulation signal. The modulation signal may be, for example, a plurality of pulse code modulated samples representing sound. The dynamic range reduction circuit modifies a selected signal to reduce the dynamic range of the sound reproduced based on the modulation signal. The selected signal is either the modulation signal or a reference signal. In one embodiment, the dynamic range reduction circuit modifies the selected signal by processing the selected signal based on a smooth saturating function. In another embodiment, the dynamic range reduction circuit modifies the amplitude of the reference signal based on an envelope size of the modulation signal.

Process and arrangement for photothermal spectroscopy

Abstract: A process and arrangements for photothermal spectroscopy (thermal wave analysis) by the single-beam method with double modulation technique. A single-beam method is developed making use of the advantages of double modulation technique in detecting the photothermally generated difference frequency without requiring partial beams and while achieving extensive absence of intermodulation, the intensity of the laser beam is modulated before striking the object in such a way that the modulation spectrum substantially contains a carrier frequency (f 1 ) and two sideband frequencies (f 1 ±F 2 ), wherein f 2 is the base clock frequency of the modulation, a regulating detector and a control loop intervening in the modulation process suppress that component of the base clock frequency (f 2 ) in the same phase with the mixed frequency of the carrier frequency and sideband frequencies. After interaction with the object the optical response of the object is measured by means of a measurement detector and frequency-selective and phase-selective device as the amplitude of that component of the base clock frequency (f 2 ) which, as the photothermal mixed product, has the same phase as the mixed frequency of the carrier frequency (f 1 ) and sideband frequency (f 1 ±f 2 ). Use for nondestructive and noncontact analysis of the material parameters of areas of solid bodies close to the surface is described.

Method and apparatus for reducing distortion in an output signal of an amplifier

Abstract: A feed forward amplifier network (100) employs a method (300) and apparatus for reducing distortion contained in an output signal (142) of an amplifier circuit (101). A modulator (118, 120) modulates (303) an input signal (141) of the amplifier circuit (101) with a reference modulation. The amplifier circuit (101) amplifies (305) the modulated signal, introducing nonlinear distortion in the process. A modulation determination circuit (107, 109, 115) determines (307) the reference modulation in the amplified signal and provides the determined modulation to an adjustment circuit (105). The adjustment circuit (131,133, 135) adjusts (309) a characteristic of the determined modulation in response to a control signal (153,155) and combines (137, 311) the adjusted modulation with the amplified signal (142) at the output of the amplifier circuit (101) to produce a corrected signal (151). A residual modulation detection circuit (139, 125, 127, 129) demodulates (313) the corrected signal (151) to recover any residual modulation remaining in the corrected signal (151) and adjusts (317) the control signal (153, 155) to reduce the recovered residual modulation.

Method and apparatus of vehicle transmission control by assured minimum pulse width

Abstract: Pulse frequency modulation is used to control brakes and clutches 10 which are operated by fluid pressure actuators 22 controlled by electrically actuated solenoid valves 28. Short pulse periods for all duty cycles are generated by feedback from the solenoid valve 28 or from the actuator 22. In one circuit an electrical control 30 triggers a flip-flop 38 which starts solenoid current. Solenoid movement results in back-emf and its effects on the solenoid flux field or current is detected and used as a feedback signal to reset the flip-flop 38 to thereby turn off the current as soon as the valve 28 is operated. In another circuit, a computer control 30' emits a command for a certain pulse period. Actuator pressure or position is monitored to produce a feedback signal to the computer. If the signal is not received, the pulse period is increased for the next pulse command so that a sufficient pulse period will be found. If the magnitude of the actuator response exceeds a threshold, the pulse period is decreased for the next pulse command. Pulse width modulation may also be improved by the same technique for minimizing the pulse period at the lowest duty cycles and yet assuring actuation.

The critical modulation frequency and its relationship to auditory filtering at low frequencies

Abstract: If the thresholds for detecting sinusoidal amplitude or frequency modulation of a sinusoidal carrier with frequency fc are expressed in terms of the respective modulation indices, m and beta, the ratio beta/m decreases as the modulation frequency increases, and approaches an asymptotic value of unity. The modulation frequency at which the ratio first becomes unity is called the critical modulation frequency (CMF). It has been suggested that the CMF is reached when the spectral sidebands in the stimulus first become detectable and that the CMF corresponds to half the value of the critical bandwidth (CB) at fc. In this paper it is demonstrated that the CMF is confounded as a measure of frequency selectivity at low frequencies, since, for modulation frequencies around the CMF, the sideband that is most detectable changes with fc. For values of fc above 250 Hz, the lower sideband is most detectable. For values of fc below 200 Hz, the upper sideband is most detectable. These findings can account for the fact that the CMF flattens off at low carrier frequencies, reaching an asymptotic value of about 40 Hz, whereas the auditory filter bandwidth continues to decrease down to very low center frequencies.

EMI reduction of power supplies by Bi-Frequency modulation

Abstract: In this paper, a new control (modulation) technique, called Bi-Frequency control, is proposed By operating the converters at two fixed switching frequencies with a modulation frequency to adjust the duration ratio, the emission spectrum of the control signal is spread and the spectral power level is reduced The spectral analysis of the Bi-Frequency control signal is given, and the spectral comparison between PWM and Bi-Frequency control is presented A Bi-Frequency controlled buck ZVS converter is breadboarded to confirm the operation of the Bi-Frequency control >

Laser fiber optic gyroscope having phase modulation

Abstract: In a laser gyroscope, a non-reciprocal phase modulation is introduced between the clockwise (CW) and counterclockwise (CCW) beams at a modulation frequency (f). The CW and CCW beams are combined on a detector and the intensity of the output is observed at the fundamental frequency (f) and the higher harmonics (e.g. 2f, 4f). At each frequency, the detected signal is demodulated by mixing it with a reference signal at the same frequency and stepping the phase of the signal through successive increments of 90°, thereby to determine the in-phase and quadrature components for each frequency monitored.

Modulation/demodulation method and system for realizing quadrature modulation/demodulation technique used in digital mobile radio system with complex signal processing

Abstract: In a complex modulation/demodulation method and system, the data frequency f C1 of data signals I and Q inputted to a constellation mapping circuit is converted by a digital interpolation circuit into a sampling frequency f C2 equal to f S /N, where f S is an operation sampling frequency and N is selected such that the signal maximum frequency of the data signals I and Q becomes lower than f C2 /2. The interpolated data signals I and Q are respectively inputted to real and imaginary input terminals of a complex coefficient band pass filter to extract a real signal output. An output signal of the complex coefficient band pass filter is converted by a DA converter into an analog signal from which desired frequency components are extracted by an analog band pass filter, thereby performing a quadrature modulation.

Time division duplex transceiver

Abstract: A transceiver for transmitting and receiving a signal having the same frequency according to a time-division includes a local oscillator for generating a local signal having a first frequency, a frequency converter for converting the local signal into a carrier signal, a modulator for modulating the carrier signal by using a transmitting baseband signal so as to generate a transmitting signal having a second frequency and a receiving section for receiving a receiving signal having the second frequency which is the same as that of the transmitting signal. The frequency convertor includes a fractional frequency convertor for converting a signal having an input frequency into a signal having an output frequency. Wherein, the output frequency is n₂/n₁ times the input frequency, where n₁ is an integer greater than 2, n₂ is an integer greater than 1, n₁ and n₂ being relatively prime, whereby the second frequency of the transmitting signal is not any integral multiple of the first frequency of the local signal. In receiving operation of the transceiver, the operation of the frequency convertor is terminated.

Orthogonal frequency division multiplex modulation signal transmission system

Abstract: PURPOSE:To send each OFDM modulation signal at a maximum bit rate per a transmission frequency band by allocating a transmission band in a form that an adjacent spectrum is in contact with spectrums of each OFDM modulation signal and locking a symbol clock phase. CONSTITUTION:At first frequency intervals DELTAf1, DELTAf2 of a carrier of 1st and 2nd OFDM modulation signals are set equal. An interval fbfa between a maximum carrier frequency fa of a 1st signal and a minimum carrier frequency fb of a 2nd signal is selected to be an integral multiple of DELTAf=DELTAf1=DELTAf2. A symbol clock phase of a 1st OFDM modulation signal and that of a 2nd OFDM modulation signal are locked to each other thereby making ideally symbol switching position of the both coincident with each other. Then the setting above is made to set a value N of NDELTAf=fb-fa to be an integer being one or over, then a DFT window applied discrete Fourier transformation to the 1st and 2nd OFDM modulation signals, then the relation of the 1st and 2nd OFDM modulation signals is set to be a relation as shown in figure. Thus, no interference disturbance is caused between them.

Spectra of data sampled at frequency-modulated rates in application to cardiovascular signals: Part 2. Evaluation of Fourier transform algorithms.

Abstract: For three direct Fourier transform algorithms we quantified the influence of pulse frequency modulation (PFM) on the spectral estimation of pulse amplitude modulation (PAM). The simulation study is based on sinusoid functions sampled according to a pulse sequence which is the output of an integral pulse frequency modulator (IPFM). One algorithm exactly reproduces the theoretical spectrum derived in Part 1. The other two, including the classical FFT, scale all PFM-induced components in a different way, and in addition, generate higher modulating frequency harmonics. For a PFM depth below 30%, the sum of spurious PFM components is almost linearly dependent on this modulation depth, for all three algorithms. Dividing the effect of PFM in a ‘harmonic’ and ‘aliasing’ distortion, we found that the FFT has a relatively high harmonic distortion, compared to an algorithm that takes into account the non-uniform character of the data. In the cardiovascular (worst) case of 30% modulation in heart rate (PFM) at a frequency of 0.1 Hz, the FFT spectrum of beat-to-beat systolic blood pressure variations contains approximately 20% of spurious components caused solely by the modulation in time occurrences of the blood pressure samples. The ‘non-uniform’ algorithm performs twice as well in this case.

Frequency modulation versus amplitude modulation discrimination: Evidence for a second frequency modulation encoding mechanism

Abstract: The encoding mechanisms for amplitude modulation (AM) and frequency modulation (FM) were investigated using AM–FM discrimination tasks In the first experiment, AM and FM were set at equally detectable levels within a trial, and discrimination thresholds were obtained adaptively in a 3IFC task Here, AM–FM discrimination thresholds were considerably larger than both AM and FM detection thresholds This is consistent with an encoding system whereby AM and FM are partially encoded by the same mechanism In the second experiment, performance on AM–FM discrimination is measured with a fixed‐level procedure Psychometric functions obtained for a constant modulation depth of AM were nonmonotonic with FMs modulation index β and each displayed a single minimum The nonmonotonic nature of the functions is consistent with a model in which FM is encoded primarily with the same mechanism that encodes AM but also with a second mechanism, probably related to changes in instantaneous frequency, that is independent of th

Phase-locked loop frequency modulation circuit for input modulation signals having low-frequency content

Abstract: A phase-locked loop frequency modulation circuit that compensates for the inability of a phase-locked loop to pass low-frequency content of an input modulation signal, and that may be utilized with existing communication apparatus, such as a cellular telephone voice radio, for accurate data transmission without having to modify such apparatus, includes a compensation circuit (50) for processing an input modulation signal (72) to provide a compensation signal (78) that is added to a loop filter output signal (70) by processing the input modulation signal to provide the same effect as adding the input modulation signal to an integrated input modulation signal (74) that is filtered by a filter (54) having the same transfer function as the loop filter (46) to provide the compensation signal.

Optical modulating and amplifying apparatus capable of achieving modulation with high signal-to-noise ratio

Abstract: An optical modulating and amplifying apparatus includes an optical amplifier and a modulator. In the apparatus, an input optical signal and an excited light are combined with each other to produce an output light signal. The excited light from a laser diode is sensed by a photodiode. The sense signal includes a bias and an amplitude to be respectively detected by a mean value sensor and a modulation signal sensor. A modulation degree of the excited light is computed by a divider. A control signal to develop a predetermined modulation degree is delivered from a control signal generator to a variable gain amplifier. In response to the control signal, the amplifier controls the amplitude of the oscillation signal from an oscillator. A laser diode driver adds a bias to the oscillation signal to drive the laser diode. The modulation degree is kept unchanged regardless of variation in an output level of the excited light so as to achieve the modulation with a high signal-to-noise ratio.

Method for measuring RF pulse frequency

Abstract: A method for measuring the frequency of a stream of RF pulses using multi-purpose, commercial-off-the-shelf test devices, such as an RF signal down converter, a digitizer and a signal processor The method is based on digital signal processing and determining the zero-crossings of the signal using signal interpolation of the pulse points The method is suitable for real-time calculations of the frequency

Apparatus for controlling the duty cycle of a pulse generator

Abstract: Apparatus is presented for controlling the duty cycle of a pulse generator which provides a periodic input signal having a frequency proportional to the rate of movement of an input member. The input signal is compared with a threshold signal and, for each cycle of the periodic signal, an ON signal is provided so long as the magnitude of the input signal exceeds that of the threshold signal. The apparatus for controlling the duty cycle includes a clock source for providing clock pulses having a fixed frequency rate. A first counter counts the total number of clock pulses occurring during an interval corresponding with N of the ON pulses to provide a total count of B. A second counter counts the total number of clock pulses occurring during each of the N ON pulses and provide a total count of A. The ratio of A to B is calculated to provide a ratio R. A new value is calculated for the threshold signal for replacing the previous value of the threshold signal wherein the new value varies as a function of the value of the ratio R.

Modulation threshlds and critical modulation frequency based on random amplitude and frequency changes

Abstract: This paper deals with determination of the critical modulation frequency (CMF) for random amplitude and frequency changes in a sinusoidal signal. The CMF is defined as the smallest modulation frequency for which AM and FM detection thresholds, expressed as modulation indices, reach the same values. The random amplitude and frequency changes were produced by modulators which were sinusoidal waves with randomly changing amplitude from period to period. The results indicate that the CMF is an increasing function of the carrier frequency and the sound pressure level. It is also shown that thresholds for detecting modulation, when expressed as root-meansquare modulation index, do not depend on the character of the modulating signal, i.e. whether this signal is periodic or random. Further, the CMF is independent of the character of the modulating signal.

High‐frequency, amplitude modulation of a submillimeter wave, medium power gyrotron

Abstract: Experimental results of the amplitude modulation of the output of a submillimeter wave gyrotron are presented. The 100% modulation of the output at frequencies up to several hundred kilohertz has been achieved with anode modulation levels of only a few percent. A good account of the experimental results is given by including a small modulation of the anode voltage in an energy transfer formula and then calculating the power output.

Phase demodulation method and apparatus for a wireless LAN, by counting the IF period

Abstract: A phase shift modulator at the transmitter has a control input connected to a binary signal source. A high frequency carrier signal is applied to a carrier signal input of the modulator. The modulated carrier signal is transmitted to a receiver where it is mixed with a local oscillator frequency. At the receiver, a modulated signal is amplified by a limit amplifier to form the received signal into square wave pulses of a uniform height. The demodulator detects when the spacing between the edges of the square wave signal change in response to the phase shift modulation at the transmitter. When the spacing between the edges of the square wave IF signal is detected to be shorter than the normal spacing for a steady IF signal with no modulation, this signifies a first binary value. A longer than normal spacing between the edges of the square wave IF signal signifies a second binary value. Frequency drift compensation circuits are disclosed to overcome frequency variations in the carrier and the local oscillator.

Method and system for indicating an optical transmit amplifier fault

Abstract: Optical transmit amplifier (24) fault indicator system (46), (78), and (80) indicates the failure of optical transmit amplifier (24) and includes an input loss circuit (46), an output failure circuit (80), and a modulation detector circuit (78). The modulation detector circuit (78) receives a portion (76) of the optical output signal (74) for determining therefrom that a failure in modulation exists in optical output signal (74) of optical transmit amplifier (24). The modulation detector (78) includes a modulation conversion circuit (82) and (84) for converting modulation in the optical output signal (76) into a measurable signal having a level within a first predetermined range in the presence of modulation and a second predetermined range in the absence of modulation. The modulation detector circuit (78) converts the measurable signal to a DC signal and produces a loss of modulation alarm signal (114) in response to a loss of modulation.

IGBT inverter with vector modulation

Abstract: This work presents the design of a three-phase inverter with insulated gate bipolar transistors (IGBTs). The pulse width modulation technique based on the space vector theory is analysed in detail. The modulation strategy considers the calculation of the on-times of the different voltage vectors that must be applied to the load, to generate the voltage vector required by the speed control system of the machine. These times are calculated previously and stored in a ROM, producing a simple, reliable, compact and low cost digital modulator. The inverter behaves as a voltage source with variable amplitude and frequency. By using the space vector theory, the number of commutations in the inverter is minimized, reaching a reduction in the switching frequency and in the current distortion. >